Selective Non-Catalytic Reduction

Selective non-catalytic reduction, or SNCR, is a method for cutting nitrogen oxide emissions downstream of a combustion process by injecting ammonia or urea into the flue gas, without using any catalyst; originally developed for coal fired power plants, it has since spread to waste incinerators, cement kilns, paper mills, metallurgical furnaces and oil refineries. The injected reagent reacts with the nitrogen oxides to form harmless nitrogen gas, carbon dioxide and water, but only within a narrow temperature window, about 1,600 to 2,000 degrees Fahrenheit for ammonia and 1,650 to 2,100 degrees for urea; outside that range the reagent either fails to react, releasing unreacted ammonia as a pollutant of its own called ammonia slip, or the ammonia oxidizes back into nitrogen oxide. In theory SNCR can remove around 90 percent of nitrogen oxides, similar to catalytic methods, but real installations typically achieve only 61 to 65 percent with ammonia and 25 to 60 percent with urea because of the difficulty of controlling temperature, timing and mixing precisely; its main advantage over selective catalytic reduction is that it needs no expensive catalyst at all.